I. Field of the Invention
The present invention relates generally to power steering systems for vehicles, and more particularly to electrically powered steering systems which include an electric drive motor for providing a powered assist to the steering gear of the host vehicle.
II. Description of the Prior Art
A variety of electrically powered steering systems (known commonly as electric power steering systems or xe2x80x9cEPS systemsxe2x80x9d) have been proposed for providing a powered assist to the steering of a motor vehicle. Conventional rack-and-pinion steering systems include a primary pinion/rack gear mesh interface for coupling the steering wheel of the vehicle to the steering system. EPS systems include an electric drive motor having a rotating element which is additionally mechanically or hydraulically coupled to the rack of the steering gear. EPS systems are said to provide fuel efficiency enhancement amounting to between about 2xc2xd and 5 percent; this enhancement is usually on the lower end of this range for relatively larger vehicles. EPS systems are also said to incorporate software which is easily programmable to provide selected steering characteristics for any particular vehicular application. However, despite overall industry developmental commitments to date on the order of a billion dollars (U.S.), no EPS system is currently offered for sale in a mass produced automobile in the United States.
There are a variety of reasons why EPS systems are not provided on automobiles in this country. One reason is that EPS systems are generally subject to an xe2x80x9cauto-steerxe2x80x9d problem, in which an unintended steering event is possible. Another reason is that EPS systems generally provide unsatisfactory tactile feedback (or xe2x80x9cfeelxe2x80x9d) during use; colloquially, EPS systems simply xe2x80x9cfeel funnyxe2x80x9d in operation. The art generally does not satisfactorily indicate the source or sources of these tactile feedback problems.
It is believed herein that the auto-steer problem occurs because the drive motor of such systems is directly linked to the host vehicle""s steering linkage and both the magnitude and the direction of steering boost are determined in an open-loop manner. Moreover, prior EPS systems appear to lack even minimal safety feedback information, such as full time independent verification that instant values of the actual assistive force have been properly generated. Errors in the proper generation of the assistive force are not sensed by the driver and remain uncorrected by the system. Other complaints about prior BPS systems include xe2x80x9cmotor cogging,xe2x80x9d lack of return ability and poor steering response to small input signals.
Several methods are known for coupling the electric drive motor of an EPS system to the steering linkage of the host vehicle. Preferred is a method comprising a hydraulic transmission for connecting the electric drive motor to the vehicle""s steering gear as disclosed in companion U.S. Provisional Patent Application Serial No. 60/090,311 entitled xe2x80x9cFeedback and Servo Control for Electric Power Steering System with Hydraulic Transmissionxe2x80x9d. The whole of that provisional patent application is expressly incorporated by reference herein. Specifically, that provisional patent application discloses connecting the electric drive motor to the vehicle""s steering gear via the motor driving a pump which, in turn, is fluidly coupled to a cylinder portion of the steering gear.
Of course, a variety of mechanical methods is known for coupling the electric drive motor of an EPS system to the steering linkage of the host vehicle. One method entails the use of a drive motor having a hollow rotor in which the rack shaft of a rack-and-pinion steering gear is concentrically disposed. The drive motor and rack shaft are connected by a ball screw and ball nut assembly, the ball screw being positioned on the rack in place of the conventional power cylinder, and the ball nut being engaged with the ball screw and supporting one end of the rotor. The other end of the rotor is supported by a thrust bearing. Other methods for coupling the drive motor to the steering linkage include coupling the drive motor to a second gear rack, via a gear train and a second pinion/rack gear mesh interface generally similar to the primary pinion/rack gear mesh interface utilized for coupling the steering wheel to the steering system, or coupling the drive motor directly to the steering shaft via a gear train. The ball screw/ball nut configuration is problematic because it couples the motor torque into the rack along with the desired axial thrust. This torque is quite sufficient to overcome the preload of the rack into the pinion so additional rotational constraint must be applied to the rack. This causes an over constraint in the gear mesh interface relationship between the pinion and the rack which results in stick-slip tactile characteristics felt at the steering wheel.
Similarly, adding a second pinion/rack gear mesh interface provides over constraint between either pinion/rack gear mesh with similar deleterious results. If the second pinion/rack gear mesh is loaded by a yoke mechanism there is additional Coulomb friction which adversely effects return ability. Coupling the drive motor directly to the steering shaft via a gear train is limited to vehicles with light steering loads because of wear limitations in the primary pinion/rack gear mesh. With either gear train, backlash becomes a tactile issue because it can be felt at the steering wheel. And if such a gear train were loaded sufficiently to eliminate the backlash, sufficient coulomb friction would be added as to adversely effect on-center feel.
Perhaps most significantly, previously known EPS systems have serious tactile problems at very low frequencies, in particular, on the general order of 1 Hz. This includes system resonance, also typical of many standard power steering systems, which is apparently ignored within the art but can readily be recognized by a driver sensing an xe2x80x9cover-centerxe2x80x9d type of instability wherein the driver must tightly grip the steering wheel in order to maintain precise control of vehicle tracking. Further exacerbating the tactile feel issue in EPS systems is another low frequency resonance traceable to the moment of inertia of the system""s electric motor. This resonance also occurs at a frequency in the general order of 1 Hz, but instead results in a xe2x80x9cnotchxe2x80x9d in the tactile characteristics.
One attempt to address some of these problems is provided in U.S. Pat. No. 5,473,539 (Shimizu et al., Dec. 5, 1995). That patent discloses an electrically operated power steering apparatus in a motor vehicle having a steering system. The apparatus comprises a steering torque detector for detecting a manual steering torque applied to the steering system, an electric motor for generating an assistive torque to be transmitted as a steering assistive force to the steering system, and an actual assistive torque detector which detects an actual assistive steering torque which is actually transmitted from the electric motor to the steering system. The apparatus also comprises a controller which generates a target value for the assistive torque to be generated by the electric motor, and which generates a control signal based on the difference between the actual assistive steering torque detected by the actual assistive torque detector and the target value, the control signal then being used to energize the electric motor.
In a first embodiment, the patent discloses an actual assistive torque detector 22 coupling the nut 11a of a ball screw mechanism 11 to a rack shaft 7 of the steering system, the assistive torque of the electric motor 10 being applied to the rack shaft 7 through the ball screw mechanism 11. The patent indicates that the actual assistive torque detector 22 may be a pressure sensor comprising a resistance wire strain gage. In a second embodiment, the detector 22 is replaced with an actual assistive torque estimator which estimates an actual assistive torque from the voltage across the electric motor and the current through it. Because the detector 22 and the estimator provide quantitative information about the magnitude of the actual assistive torque, quantitative information which is necessary for the rest of the disclosed parts of the system to act in the manner described in the patent, the detector 22 and estimator do more than merely xe2x80x9cdetectxe2x80x9d or respond to the presence or absence of an actual assistive torque; instead, they actually measure or estimate its value. A third embodiment in the patent attempts to give the driver of the vehicle a comfortable feel of steering action by providing a high- and/or a low pass filter in the actual assistive torque detector. The disclosed purpose of such filters is to reduce the purported noise from harmonics which are generated in the system upon differentiation for conversion from a rotational angular speed into a rotational angular acceleration, such that the estimator takes into account the inertial torque and the viscosity torque with respect to the motor torque within the system, based on the motor current and the rotational angular speed.
This patent appears to reflect a belief throughout the automotive industry that the issue of poor tactile feedback can and should be addressed by increasingly complicated software control schemes wherein the applied steering boost is made to model the input steering effort. Such efforts, however, have lead to enormous development expenditures without commensurate results; the art appears to provide no guidance as to actually solving the low frequency problems described above.
The related problem of steering shudder was addressed by the method and apparatus for enhancing stability in servo systems disclosed in U.S. Pat. No. 5,544,715 (E. H. Phillips, Aug. 13, 1996). The whole of that patent is expressly incorporated by reference herein. The patent discloses the use of series damping devices to form compliant couplings having series damping characteristics, used either for mounting hydro-mechanically driven actuators, or for coupling them to load elements which they position. The series damping absorbed sufficient energy to provide adequate gain and phase margins for the feedback characteristics of systems utilizing such actuators, so as to substantially prevent the occurrence of high frequency shudder. Of particular interest are the general steering system characteristics described in the specification of the ""715 patent and depicted in the block diagram shown in FIG. 3 of that patent. The ""715 patent discloses mechanical devices and methods for achieving servo control of the open-loop feedback characteristics present in general steering systems.
Many prior EPS systems appear to experience only marginally stable control and suffer a resultant amplification of external disturbances to them. While general techniques for achieving servo control in other systems are discussed in a variety of textbooks, the application of such techniques to EPS systems would require a knowledge (presently not possessed in the art) of precisely where undesired resonances arise in EPS systems. More particularly, an introduction to servo control which can easily be understood by a novice in this field can be found in a xe2x80x9ccribxe2x80x9d text book by DiStefano, Stubberud, and Williams entitled Schaum""s Outline of Theory and Problems of Feedback and Control Systems and published by the McGraw-Hill Book Company. As pointed out in that book, any servo system having a feedback loop can oscillate via self excitation at any frequency whereat unity gain in the opened feedback loop coincides with an odd multiple of 180xc2x0 phase shift of that opened feedback loop""s phase angle. The prior EPS systems mentioned above appear to be characterized by the near confluence of these conditions, with the resulting marginally stable control and amplification of external disturbances mentioned above.
The block diagram shown in FIG. 3 of the ""715 patent is both complex and complicated, and discloses several feedback paths inherent in a variety of steering systems. However, block diagrams like that shown in FIG. 3 of the ""715 patent can be reduced via appropriate algebraic manipulation to substantially simpler diagrams like that shown in FIG. 4 of the ""715 patent. By such algebraic manipulation, all of the forward gain factors can be considered as being comprised within a single forward gain block xe2x80x9cGxe2x80x9d, while all of the feedback gain factors can be considered as being comprised within a single feedback gain block xe2x80x9cHxe2x80x9d.
The closed loop gain ratio O/I of a system whose analysis is reduced in this manner can be determined by the formula:       O    I    =      G          1      +      GH      
wherein O is a particular output value of the system, I is a particular input value for the system, G is the forward gain value and H is the feedback gain value. It should be readily apparent that the closed loop gain ratio O/I becomes unstable at any frequency or frequencies where the open loop gain GH attains a value of minus 1, that is, where the absolute value of GH has a value of 1 and its phase angle is equal to an odd multiple of 180xc2x0; the denominator of the ratio O/I rapidly approaches zero, so that the gain rapidly approaches infinity. Similarly, the closed loop gain ratio O/I is at best only marginally stable at any frequency or frequencies where the open loop gain GH attains a value which is close to (but not equal to) minus 1. Failure to counteract or otherwise address any particular resonance associated with an open loop gain GH value close to minus 1 would result in such a system having marginal stability. Unfortunately, one such shortcoming appears to be typical of prior EPS systems, since it appears that the art as a whole has failed to recognize or correctly analyze the source of the resonance which causes the low frequency stability control problems mentioned above.
It would be highly advantageous to provide methods and apparatus for substantially eliminating stability problems in EPS systems, and, in particular, for substantially eliminating the low frequency stability control problems mentioned above. It would also be highly advantageous to provide full time independent verification of instant values of steering boost in EPS systems in order to substantially eliminate concerns relating to auto-steer. It would also be advantageous for such a system to achieve a substantially linear control relationship between an applied steering torque input and a resulting steering force output, and thereby achieve an optimum tactile relationship between a vehicle, the vehicle driver and the steering system of the vehicle. It would be still further advantageous for such a system to operate in a regenerative manner and enjoy all of the benefits of a regenerative system.
These and other objects are achieved in EPS systems according to the present invention, in which feedback signals are either derived from a calculated torque signal, generated torque signals or a generated force signal, which signals are representative either of steering assist force or of total steering force, and use the derived feedback signals in a feedback manner so as to provide servo control over the steering force output of the systems. Such control is implemented over electric motors in preferred, and first, second and third alternate preferred embodiments of the present invention via impedance matching means for providing assisted steering. In the preferred and first alternate preferred embodiments speed reduction means provide a controlled steering assist component only, while in the second and third alternate preferred embodiments torque summing means and force generating means respectively provide controlled assisted steering comprising mechanical steering force components as well as steering assist force components.
The present invention resides, in part, in the recognition that the various problems mentioned above with regard to EPS systems are feedback control issues which are better addressed via appropriate hardware and servo control, rather than by more complex software control as has been done in prior EPS systems. The present invention also resides, in part, in the recognition that the reflected moment of inertia of the electric motor of such systems is in resonance with the series combination of various spring-like elements included in the steering load path. More particularly, it is believed herein that a primary reason for such tactile feedback problems is a low frequency resonance between the reflected moment of inertia of such a system""s electric motor and the spring-like steering load itself. As may be more clearly understood with reference to the block diagram shown in FIG. 3 of the ""715 patent and the associated discussion in the specification of that patent, the most compliant one of these spring-like elements (and therefore the dominant one of them) is generally formed by system compliance beyond the dirigible wheels. Depending upon vehicle speed, this compliance is formed by a parallel relationship between tire sidewall stiffness and tire patch loading characteristics. For most non-zero vehicle speeds, the dominant system compliance (i.e., the inverse of stiffness) is related to the product of centrifugal force and caster offset. The EPS systems of the present invention simply treat whichever combination yields such compliance as a spring, and generate a servo-controlled steering assist force in a stable manner for providing a powered assist to steering the host vehicle.
The power steering systems of the present invention include a primary applied steering torque sensor that generates an applied steering torque signal VTT1 and supplies that signal to an electronic control means. The electronic control means processes that signal and utilizes it in an internal feedback loop comprising the impedance matching means for selectively generating the steering assist force respectively comprised in the preferred, first alternate preferred, second alternate preferred and third alternate preferred embodiments of the present invention. The electronic control means compensates the applied steering torque signal VTT1, obtains a tachometer signal Vt (in a manner described in more detail below) and subtracts the tachometer signal Vt from the compensated torque signal, yielding an internal feedback loop input signal Vi. The electronic control means further obtains a steering assist force dependent internal loop feedback signal Vf (again, in any of the four embodiments described in more detail below) and subtracts the internal loop feedback signal Vf from the internal feedback loop input signal Vi, yielding an error signal Ve. The electronic control means then determines an internal control gain value ICG dependent upon the instant applied steering torque signal VTT1, the speed of the host vehicle and other desirable parameters, and multiplies the error signal Ve by the internal control gain value ICG to yield an internal error signal Ves. The electronic control means amplifies the internal error signal Ves to yield an internal drive signal Vd, which the electronic control means uses to operate the electric motor. The electric motor is directly coupled to the impedance matching means. A calculated net torque provides an assist torque signal VTT2 in the preferred embodiment of the present invention. On the other hand, an assist torque signal VTT2xe2x80x2 is provided by a torque sensor in the first alternate preferred embodiment. And, torque and force sensors generate torque and force signals VTT6 and VFT1, respectively utilized in the second alternate preferred and third alternate preferred embodiments. These signals are indicative of either the appropriate torque Ta or Tao, or force Fr value attained by the transmission means wherein Ta refers to assist torque values attained in the preferred and first alternate preferred embodiments, Tao refers to total torque, or assisted output torque values attained in the second alternate preferred embodiment, and Fr refers to steering rack output forces attained in the third alternate preferred embodiment of the present invention. Finally, the torque or force value attained by the impedance matching means is multiplied by a feedback factor comprising the inverse of the internal control gain value ICG to form the internal, torque or force-dependent loop feedback signal Vf which is fed back for subtraction from Vi to yield the error signal Ve.
The above mentioned calculated net torque can be determined from the electric motor input voltage and current as follows: Calculated motor air gap torque is directly determined from the product of the motor""s torque constant and the input current. Calculated rotor accelerating torque is determined from the product of the rotor inertia and differential of the motor""s rotational speed (the rotational speed having a rad./sec. dimension) with respect to time. The motor""s rotational speed is determined form the product of the motor""s back EMF and the inverse of the motor""s voltage constant. The motor""s back EMF, in turn, is determined by subtracting the I R drop from the input voltage, where I is the motor current an R is the motor resistance. The calculated net torque then, is simply the difference between the calculated motor air gap torque and the calculated rotor accelerating torque.
The power steering systems of the present invention thus yield controlled torque or force Ta, Tao or Fr values which, other than for very short term effects such as during the rotor acceleration mentioned above, are linearly related to the product of the instant applied steering torque signal VTT1 and the instant control gain value ICG. In this manner, the power steering systems of the present invention isolate the electric motor within its own internal feedback loop. The input to the internal loop is linearly related to the applied steering torque signal VTT1 generated by the applied steering torque sensor, and the output is the torque or force Ta, Tao or Fr. As mentioned above, the torque Ta may either be a calculated value or measured by a torque sensor while the torque or force Tao or Fr is necessarily measured by a torque or force sensor. The internal loop functions such that the moment of inertia of the rotor of the electric motor is effectively decoupled from the overall control loop. This generates an optimum tactile relationship between a vehicle, the vehicle""s driver and the steering system of the vehicle.
It is highly preferred that the power steering systems of the present invention include a redundant applied steering torque sensor. Such a redundant steering torque sensor prevents system runaway in case the primary applied steering torque sensor should fail in such a way as to give a fixed, non-zero value to the applied steering torque Ts. Since an indication of merely the magnitude of the applied steering torque might be adequate for this purpose, the redundant applied steering torque sensor could be the sensor disclosed in co-pending Provisional U.S. patent application Ser. No. 60/070,732 entitled xe2x80x9cAdjustable, Preloaded Transducer, Especially in a Sensor for Measuring Applied Steering Torquexe2x80x9d (E. H. Phillips, filed Jan. 7, 1998). The whole of that provisional patent application is expressly incorporated by reference herein. Preferably, however, the redundant applied steering torque sensor is constructed in the same manner as, and operates on the same principle as, the primary applied steering torque sensor.
Also preferably, the electronic control means and electric motor in the systems of the present invention are capable of handling regenerative electric power so as to enable the recovery of power returned to the system whenever the steering load actively centers the steering rack. In general, this requires the impedance matching means to transmit power in either direction.
As described above, the electric motor drives the impedance matching means so as to provide torque or force in a directionally servo controlled manner with reference to the tachometer feedback information and the torque or force-dependent loop feedback signal Vf. One control problem arising from the use of conventional torque or force sensors for this purpose is the need to calibrate them with reference to zero value. Advantageously, torque sensors for this purpose are constructed according to the teachings of U.S. Pat. No. 5,351,555 by I. J. Garshelis entitled xe2x80x9cCircularly Magnetized Non-Contact Torque Sensor and Method for Measuring Torque Using Samexe2x80x9d because such devices inherently tend to have a zero output when unstressed. Thus, such devices are preferably utilized for measuring Ta and Tao.
Otherwise (and possibly in addition to utilizing such devices), zero drift can be suppressed by passing the sensor output signal through a high pass filter having a very low comer frequency. Such a high pass filter can be implemented in software as an equivalent to a series capacitor followed by a parallel resistor to ground. The equivalent transfer function for this circuit is s/(xcfx89c+s) where s is the first-order Laplace variable and xcfx89c is the desired filter comer frequency in rad./sec. The Laplace variable s is also referred to as the complex-frequency variable; as explained in detail in chapters 15 through 17 of H. H. Skilling, Electrical Engineering Circuits (John Wiley and Sons, Inc., New York, N.Y., 1957). The time constant associated with this circuit is 1/xcfx89c. In configuring such a high pass filter, this time constant should be made longer than any expected steering event (i.e., such as using a time constant of 100 sec.).
In a first aspect, then, the preferred embodiment of the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a speed reduction means having input and output nodes, and adapted to supply a powered assist to steering the dirigible wheels of the vehicle via torque delivered from the output node upon the supply of torque to the input node; a primary applied steering torque sensor which generates an applied torque signal VTT1 in response to at least an applied steering torque Ts; means for calculating the assist torque Ta delivered from the output node which generates a feedback signal Vf; an electric motor operatively connected to and capable of reversibly driving the speed reduction means via the input node; and an electronic control means to which the primary applied steering torque sensor is operatively connected, and which utilizes the applied torque signal VTT1 and the feedback signal Vf as input information for controlling actuation of the electric motor; wherein the electronic control means establishes servo control over the powered assist to steering supplied by the speed reduction means in dependence upon the feedback signal Vf representative of the assist torque Ta calculated by the electronic control means.
In a second aspect, then, the first alternate preferred embodiment of the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a speed reduction means having input and output nodes, and adapted to supply a powered assist to steering the dirigible wheels of the vehicle via torque delivered from the output node upon the supply of torque to the input node; a primary applied steering torque sensor which generates an applied torque signal VTT1 in response to at least an applied steering torque Ts; a primary assist torque sensor sensing the assist torque Ta delivered from the output node which generates a feedback signal Vf; an electric motor operatively connected to and capable of reversibly driving the speed reduction means via the input node; and an electronic control means to which the primary assist torque sensor and the primary applied steering torque sensor are operatively connected, and which utilizes the applied torque signal VTT1 and the feedback signal Vf as input information for controlling actuation of the electric motor; wherein the electronic control means establishes servo control over the powered assist to steering supplied by the speed reduction means in dependence upon the feedback signal Vf representative of the assist torque Ta sensed by the primary assist torque sensor.
In a third aspect, the second alternative preferred embodiment of the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a torque summing means having applied torque input, electric motor input and output nodes, and adapted to supply assisted output torque for steering the dirigible wheels of the vehicle upon the supply of torque to the input node; a primary applied steering torque sensor which generates an applied torque signal VTT1 in response to at least an applied steering torque Ts; a primary output torque sensor sensing the assisted output torque Tao delivered from the output node; an electric motor operatively connected to and capable of reversibly driving the torque summing means via the electric motor input node; and an electronic control means to which the primary output torque sensor and the primary applied steering torque sensor are operatively connected, and which utilizes the applied torque signal VTT1 and the feedback signal Vf as input information for controlling actuation of the electric motor; wherein the electronic control means establishes servo control over the assisted output torque for steering the dirigible wheels supplied by the torque summing means in dependence upon the feedback signal Vf representative of the assisted output torque Tao sensed by the primary output torque sensor.
In a fourth aspect, the third alternative preferred embodiment of the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a steering force generating means having applied torque input, electric motor input and steering force output nodes, and adapted to supply assisted steering force for steering the dirigible wheels of the vehicle upon the supply of torque to the input node; a primary applied steering torque sensor which generates a signal VTT1 in response to at least an applied steering torque Ts; a primary steering force sensor sensing the steering force Fr delivered from the steering force output node; an electric motor operatively connected to and capable of reversibly driving the steering force generating means via the electric motor input node; and an electronic control means to which the primary steering force sensor and the primary applied steering torque sensor are operatively connected, and which utilizes the applied torque signal VTT1 and the feedback signal Vf as input information for controlling actuation of the electric motor; wherein the electronic control means establishes servo control over the assisted steering force for steering the dirigible wheels supplied by the steering force generating means in dependence upon the feedback signal Vf representative of the steering force Fr sensed by the primary output torque sensor.
In any of these systems, the electronic control means of the system establishes closed loop servo control over the electric motor in dependence upon the torque or force Ta, Tao or Fr, calculated or sensed by the appropriate one of the primary assist torque, assisted output torque or steering force sensors, and achieves a substantially linear relationship between the applied steering torque Ts and the torque or force Ta, Tao or Fr, supplied by the speed reduction, torque summing or steering force generating means, respectively. Preferably, the electronic control means generates an error signal Ve in response to at least the generation of the applied torque signal VTT1 by the primary applied steering torque sensor, the error signal Ve being dependent upon the torque or force Ta, Ta or Fr, calculated or sensed by the appropriate one of the primary assist torque, assisted output torque or steering force sensors. The electronic control means determines an internal control loop gain value ICG as a function of at least vehicle speed, the internal control loop gain value ICG being further related to the applied torque signal VTT1 generated by the primary applied steering torque sensor, multiplies the error signal Ve by the internal control loop gain value ICG to generate an internal error signal Ves which is amplified to form an internal drive signal Vd and causes the electric motor to drive the speed reduction, torque summing or steering force generating means in a manner which generates the torque or force Ta, Tao or Fr, sensed by the appropriate one of the primary assist torque, assisted output torque or steering force sensors in accordance with internal error signal Ves. More preferably, the electronic control means 32 generates the error signal Ve by compensating the signal VTT1 generated by the primary applied steering torque sensor to yield a compensated steering torque signal Vc, subtracting from the compensated steering torque signal Vc a tachometer signal Vt, (representative of steering movement of the dirigible wheels) to yield an input signal Vi, obtaining a torque or force-dependent loop feedback signal Vf and subtracting the feedback signal Vf from the input signal Vi to yield the error signal Ve.
It should be clear that, in none of these aspects of the invention (as well as in the fifth, sixth, seventh and eighth aspects of the invention described below) no target value for any system parameter is ever established at all for control purposes. This is completely unlike the system disclosed in the Shimizu et al. ""539 patent.
The power steering systems of these aspects of the present invention preferably further comprise an actuation speed calculation means operatively comprised within the electronic control means for providing the tachometer signal Vt representative of steering movement of the dirigible wheels. The electronic control means subtracts the tachometer signal Vt so provided from the compensated steering torque signal Vc during control of actuation of the electric motor. As mentioned above, the actuation speed calculating means can conveniently comprise calculation of the rotational speed of the electric motor via the product of the motor""s back EMF and the inverse of the motor""s voltage constant, or, alternately a tachometer can be utilized for measuring the rotational speed of at least one of the vehicle""s steering wheel and the electric motor.
The power steering systems of the present invention preferably comprise redundant measures for confirming proper operation of the system, its component elements and the electronic control means. For example, each of the systems preferably comprises a redundant applied steering torque sensor which is connected to the electronic control means and which generates a redundant signal VTT3 in response to the applied steering torque Ts. The electronic control means then terminates the powered assist provided by the system when the redundant signal VTT3 from the redundant applied steering torque sensor fails to correlate with the signal VTT1 from the primary applied steering torque sensor.
Similarly, the power steering system of the preferred embodiment of the present invention preferably further utilizes the electronic control means for calculating an expected but fictitious assist torque signal VTT4 as a function of the signal VTT1 generated by the primary applied steering torque sensor; compares the instant value of the calculated assist torque signal VTT2 with the expected but fictitious assist torque signal VTT4; and terminates the powered assist provided by the system when the calculated assist torque signal VTT2 fails to correlate with the expected but fictitious assist torque signal VTT4. It is important to note that this calculation of an expected but fictitious assist torque signal VTT4 is not used as a target for controlling the electric motor, but only for generating the feedback signal Vf.
Also similarly, each of the power steering systems of the first, second and third alternate preferred embodiments of the present invention preferably farther comprises a means for providing a redundant measure of the torque or force Ta, Tao or Fr. The means for providing a redundant measure of the torque or force Ta, Tao or Fr conveniently comprise like sensors to those sensors used for providing the primary measurement of the torque or force Ta, Tao or Fr. The means for providing a redundant measure of the torque or force Ta, Tao or Fr respectively generate a redundant assist torque signal VTT5, a redundant assisted output torque signal VTT8, or a redundant steering force signal VFT3. As before, the electronic control means terminates the powered assist provided by the system when the redundant measure of the torque or force Ta, Tao or Fr fails to correlate with the torque or force Ta, Tao or Fr sensed by the appropriate one of the primary assist torque, assisted output torque or steering force sensors.
More preferably, in each of the power steering systems of the first, second and third alternate preferred embodiments of the present invention the electronic control means calculates an expected but fictitious assist torque signal VTT4, assisted output torque signal VTT7, or steering force signal VFT2 as a function of the signal VTT1 generated by the primary applied steering torque sensor; compares the signal VTT2, VTT6 or VFT1, and the redundant signal VTT4, VTT8 or VFT3 provided by the means for providing the same, to the expected but fictitious signal VTT4, VTT7 or VFT2; and terminates the powered assist provided by the system when either the primary, or redundant measure of the torque or force Ta, Tao or Fr fails to correlate with the appropriate expected but fictitious signal VTT4, Vt or Vt calculated by the electronic control means. It is important to note that this calculation of an expected but fictitious signal VTT4, VTT7 or VFT2 is not used as a target for controlling the electric motor, but only for indicating when the system is not operating properly.
Preferably, each of the systems also comprises clutch means, or functionally clutch-like means, for disconnecting the electric motor and as much of the impedance matching means as possible from remaining portions of the power steering system should the powered assist to steering provided by the system fail. Also preferably, the clutch means, or functionally clutch-like means is spring-biased to achieve such disconnection in a fail-safe manner if the system should fail. (xe2x80x9cFailxe2x80x9d includes actual system failure or the intentional termination of the powered assist provided by the system.)
The power steering system is preferably a regenerative system similar to that disclosed in co-pending Provisional U.S. patent application Ser. No. 60/073,560, entitled xe2x80x9cRegenerative Power Steering System Including Solenoid-Actuated Four-Way Cross-Over Valvexe2x80x9d (E. H. Phillips, filed Feb. 3, 1998. The whole of that provisional patent application is expressly incorporated by reference herein. Thus, it is preferred that the system of the present invention is regenerative and alternatively operable in an unassisted straight steering mode, a powered assist mode in which the powered assist is provided by the electric motor during entry of the vehicle into a turn and an energy regeneration mode which recovers energy while the vehicle exits a turn. The electronic control means controls the electric motor so as to provide torque to the input or electric motor input node during the powered assist mode of the system, and permit the electric motor to generate energy recovered through the speed reduction, torque summing or steering force generation means via reversed torque during the energy regeneration mode of the system. One advantageous feature of the system is that the electric motor does not rotate during the straight steering mode of the system. The system includes an electrical energy means for storing and delivering electrical power, such that electrical energy is delivered from the electrical energy means to the electric motor during the powered assist mode of the system, and is stored to the electrical energy means by the electric motor during the energy regeneration mode of the system.
The electronic control means of any of the systems preferably employs signals representative of the torque or force Ta, Tao or Fr respectively calculated or provided by the primary assist torque, assisted output torque or steering force sensor to counteract a resonance occurring predominantly between the moment of inertia of the rotor of the electric motor and a system spring rate presented by the steering load stiffness of the host vehicle. More particularly, the electronic control means treats the resonance as a resonance between a mass element predominated by the moment of inertia of the rotor of the electric motor and a theoretical spring element dominated by tire loading characteristics associated with the tires. The electronic control means then employs the torque or force signal VTT2, VTT2xe2x80x2, VTT7 or VFT2 respectively calculated or provided by the primary assist torque sensor, assisted output torque sensor or steering force sensor to control the electric motor so that the speed reduction, torque summing or steering force generating means is not driven in harmony with that resonance, or any other control system generated resonance. In particular, the electronic control means employs the signals representative of the torque or force Ta, Tao or Fr within an internal servo control loop comprising the electric motor and the speed reduction, torque summing or steering force generating means to control the resulting torque or force Ta, Tao or Fr as a selected function of the torque control signal according to the internal gain value ICG. Thus, the operation of the electric motor is controlled such that the speed reduction, torque summing or steering force generating means is not driven in harmony with that resonance, or any other control system generated resonance.
The electronic control means preferably provides for speed sensitive steering by reducing the internal gain value ICG as a selective function of vehicle speed. In addition, the electronic control means preferably provides compensation of at least one of the magnitude of the applied steering torque, the operation of the electric motor, and the torque or force signals VTT2, VTT2xe2x80x2, VTT7 or VFT2 calculated or measured by the appropriate one of the primary assist torque, assisted output torque or steering force sensors. More preferably, the compensation allows for stable operation of both the internal servo control loop and the overall system servo control loop with adequate phase and gain margins over the entire range of vehicle speed and other operational factors, such as a driver abruptly encountering glare ice on an otherwise dry roadway, and the like. In general, such operation requires selective manipulation of the compensation via selective variation of the comer frequencies of poles and/or zeros comprised in the compensation with respect to vehicle speed, in order to adjust for changes in the internal gain value ICG and the spring-like steering load. Preferably, the compensation of the magnitude of the applied steering torque predominantly or exclusively includes at least one pole, while the compensation of the internal servo control loop, between the input signal Vi and the steering force assist ultimately supplied to the dirigible wheels by the system, is predominated by a low-frequency zero.
This latter compensation may be carried out either in the forward direction by compensating the internal error signal Ves, or in the feedback path. It has been found preferable to confine such the compensation of the internal servo control loop to the feedback path. As will be explained below, this is because all of the perturbing disturbance factors are present in the feedback path but are not yet directly present in the product of the internal gain value ICG and the error signal Ve which forms the internal error signal Ves.
The electronic control means also preferably employs the torque or force signals VTT2, VTT2xe2x80x2, VTT7 or VFT2 respectively calculated or provided by one of the primary assist torque, assisted output torque or steering force sensors in establishing the internal servo control loop. Vi is derived from the application of an applied steering torque Ts to the steering wheel of the host vehicle. More particularly, the system preferably comprises either motor rotational speed calculation or a motor driven tachometer operatively connected to the electronic control means, which generates a signal Vt representative of the rotational speed of the electric motor. The electronic control means subtracts Vt from the compensated signal Vc (generated together by the primary applied steering torque sensor and the electronic control means) and employs the resulting difference as the input signal Vi to the internal servo control loop. Vt is equal to xcex8m s/Kc, xcex8m being a rotational position of the electric motor, Kc being a tachometer feedback damping factor and s being the first-order Laplace variable. If the functions at issue are linear (as they are here), the first-order Laplace variable reduces to the first order derivative of those functions with respect to time. Herein, s2 refers to the second-order Laplace variable, which similarly reduces to the second order derivative with respect to time. The reciprocals 1/s and 1/s2 reduce respectively to single and double integration with respect to time.
Once the electronic control means generates the input signal Vi to the internal servo control loop, the electronic control means further generates (and preferably compensates) a signal representative of the torque or force Ta, Tao or Fr respectively calculated or provided by one of the primary assist torque, assisted output torque or steering force sensors so as to give an internal loop feedback signal Vf, and subtracts the internal loop feedback signal Vf from the input signal Vi to the internal servo control loop so as to yield an error signal Ve from which a drive signal Vd for operating the electric motor is derived. The electronic control means further determines an internal control loop gain value ICG as a function of vehicle speed and the signal VTT1, and operates the electric motor so as to generate a calculated or measured torque or force signal VTT2, VTT2xe2x80x2, VTT7 or VFT2 in accordance with an internal error signal Ves equal to the product of the error signal Ve and the internal control loop gain value ICG. (Alternately, compensation could be applied to that product to yield an internal modified error signal Vmes that would then be used instead of Ves.)
In a fifth aspect, a preferred embodiment of the present invention is directed to a particular combination of the elements identified above. More particularly, in this fifth aspect, the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a speed reduction means having an input node and an output node and adapted to supply a powered assist to steering the dirigible wheels of the vehicle upon the application of torque to the input node; a primary applied steering torque sensor which generates a signal VTT1 in response to at least an applied steering torque Ts; an electric motor operatively connected to and capable of reversibly driving the speed reduction means; a calculation of net torque delivered to the speed reduction means by the electric motor which thereby generates a value for, an assist torque signal VTT2 related to assist torque Ta delivered from the output node; an electronic control means for calculating the net torque and to which the primary applied steering torque sensor is operatively connected, and which controls actuation of the electric motor; a redundant applied steering torque sensor which generates a redundant applied torque signal VTT3 in response to the applied steering torque Ts and which is connected to the electronic control means; an actuation speed calculation means operatively comprised within the electronic control means for providing a tachometer signal Vt representative of steering movement of the dirigible wheels; a calculated expected but fictitious value for an expected assist torque signal VTT4 indicative of expected assist torque; wherein the electronic control means establishes servo control over the powered assist to steering supplied by the speed reduction means in dependence upon the assist torque signal VTT2 generated by the calculated value therefore; wherein the electronic control means establishes closed loop servo control over the electric motor in dependence upon the assist torque signal VTT2 generated by the calculated value therefore and achieves a substantially linear relationship between the applied steering torque Ts and the powered assist to steering supplied by the speed reduction means; wherein the electronic control means generates an error signal Ve in response to at least the generation of the applied torque signal VTT1 by the primary applied steering torque sensor, the error signal Ve being dependent upon the assist torque signal VTT2 generated by the calculated value therefore; wherein the electronic control means determines an internal control loop gain value ICG as a function of at least vehicle speed, the internal control loop gain value ICG being further related to the signal VTT1 generated by the primary applied steering torque sensor; and wherein the electronic control means multiplies the error signal Ve by the internal control loop gain value ICG and causes the electric motor to drive the speed reduction means in a manner which generates an assist torque Ta in accordance with the product of the error signal Ve and the internal control loop gain value ICG; wherein the electronic control means generates the error signal Ve by compensating the signal VTT1 generated by the primary applied steering torque sensor to yield a compensated steering torque signal Vc; subtracting from the compensated steering torque signal Vc a tachometer signal Vt, representative of steering movement of the dirigible wheels, to yield an input signal Vi; obtaining an assist torque-dependent internal loop feedback signal Vf and subtracting the feedback signal Vf from the input signal Vi to yield the error signal Ve; wherein the electronic control means terminates the powered assist provided by the system when the redundant applied torque signal VTT3 from the redundant applied steering torque sensor fails to correlate with the applied torque signal VTT1 from the primary applied steering torque sensor; wherein the vehicle includes a steering wheel to which the primary applied steering torque sensor is operatively connected, and wherein the actuation speed calculation means calculates the rotational speed of the electric motor; wherein the electronic control means terminates the powered assist provided by the system when the expected but fictitious assist torque signal VTT4 indicative of expected assist torque Ta fails to correlate with the assist torque signal VTT2 generated by the calculated value therefore which is indicative of the actual assist torque Ta; wherein the system is regenerative and alternatively operable in an unassisted straight steering mode, a powered assist mode in which the powered assist is provided by the speed reduction means during entry of the vehicle into a turn and an energy regeneration mode which recovers energy while the vehicle exits a turn; and wherein the electronic control means controls the electric motor so as to provide same direction rotative application of torque to the input node during the powered assist mode of the system, and permits the electric motor to generate energy recovered through the speed reduction means from a counter direction rotative application of torque during the energy regeneration mode of the system; wherein the vehicle presents a steering load stiffness to the power steering system; and wherein the electronic control means employs the calculated assist torque signal VTT2 indicative of the actual assist torque Ta to counteract an electromechanical resonance occurring predominantly between the moment of inertia of the rotor of the electric motor and a system spring rate presented by the steering load stiffness; wherein the electronic control means treats the electromechanical resonance as a resonance between a mass element predominated by the moment of inertia of the rotor of the electric motor and a theoretical spring element dominated by tire loading characteristics associated with the tires; and wherein the electronic control means employs the calculated assist torque signal VTT2 indicative of the actual assist torque Ta to control the electric motor so that the speed reduction means is not driven in harmony with the electromechanical resonance; wherein the electronic control means provides compensation of at least one of the magnitude of the applied steering torque Ts and the operation of the electric motor or the assist torque signal VTT2, and wherein the compensation of the magnitude of the applied steering torque Ts predominantly includes at least one pole and the compensation of the operation of the electric motor or the assist torque signal VTT2 is predominated by a low-frequency zero; wherein the vehicle includes a steering wheel, such that the application of an applied steering torque Ts to the steering wheel results in the application by the system of a steering force assist to the dirigible wheels; wherein the electronic control means employs the calculated assist torque signal VTT2 indicative of the actual assist torque Ta to establish an internal servo control loop between the input signal Vi derived from the application of an applied steering torque Ts to the steering wheel and the steering force assist to the dirigible wheels, and wherein the speed reduction means and electric motor are disconnected from the remaining portions of the power steering system should the powered assist to steering provided by the system fail.
In a sixth aspect, a first alternate preferred embodiment of the present invention is directed to another particular combination of the elements identified above. More particularly, in this sixth aspect, the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a speed reduction means having an input node and an output node and adapted to supply a powered assist to steering the dirigible wheels of the vehicle upon the application of torque to the input node; a primary applied steering torque sensor which generates a signal VTT1 in response to at least an applied steering torque Ts; a primary assist torque sensor which generates an assist torque signal VTT2xe2x80x2 in response to assist torque Ta delivered from the output node; an electric motor operatively connected to and capable of reversibly driving the speed reduction means; an electronic control means to which the primary assist torque sensor and the primary applied steering torque sensor are operatively connected, and which controls actuation of the electric motor; a redundant applied steering torque sensor which generates a redundant applied torque signal VTT3 in response to the applied steering torque Ts and which is connected to the electronic control means; an actuation speed calculation means operatively comprised within the electronic control means for providing a tachometer signal Vt representative of steering movement of the dirigible wheels; a calculated expected but fictitious value for an expected assist torque signal VTT4 indicative of expected assist torque; a redundant assist torque sensor for providing a redundant assist torque signal VTT5 indicative of the actual assist torque; wherein the electronic control means establishes servo control over the powered assist to steering supplied by the speed reduction means in dependence upon the assist torque signal VTT2 generated by the primary assist torque sensor; wherein the electronic control means establishes closed loop servo control over the electric motor in dependence upon the assist torque signal VTT2 generated by the primary assist torque sensor and achieves a substantially linear relationship between the applied steering torque Ts and the powered assist to steering supplied by the speed reduction means; wherein the electronic control means generates an error signal Ve in response to at least the generation of the applied torque signal VTT1 by the primary applied steering torque sensor, the error signal Ve being dependent upon the assist torque signal VTT2 generated by the primary assist torque sensor; wherein the electronic control means determines an internal control loop gain value ICG as a function of at least vehicle speed, the internal control loop gain value ICG being further related to the signal VTT1 generated by the primary applied steering torque sensor; and wherein the electronic control means multiplies the error signal Ve by the internal control loop gain value ICG and causes the electric motor to drive the speed reduction means in a manner which generates an assist torque Ta at the primary assist torque sensor in accordance with the product of the error signal Ve and the internal control loop gain value ICG; wherein the electronic control means generates the error signal Ve by compensating the signal VTT1 generated by the primary applied steering torque sensor to yield a compensated steering torque signal Vc; subtracting from the compensated steering torque signal Vc a tachometer signal Vt, representative of steering movement of the dirigible wheels, to yield an input signal Vi; obtaining an assist torque-dependent internal loop feedback signal Vf and subtracting the feedback signal Vf from the input signal Vi to yield the error signal Ve; wherein the electronic control means terminates the powered assist provided by the system when the redundant applied torque signal VTT3 from the redundant applied steering torque sensor fails to correlate with the applied torque signal VTT1 from the primary applied steering torque sensor; wherein the vehicle includes a steering wheel to which the primary applied steering torque sensor is operatively connected, and wherein a tachometer redundantly measures the rotational speed of at least one of the steering wheel and the electric motor; wherein the electronic control means terminates the powered assist provided by the system when either the calculated expected by fictitious assist torque signal VTT4 or the redundant assist torque signal VTT5 indicative of the actual assist torque Ta fails to correlate with the assist torque signal VTT2 generated by the primary assist torque sensor also indicative of the actual assist torque Ta; wherein the system is regenerative and alternatively operable in an unassisted straight steering mode, a powered assist mode in which the powered assist is provided by the speed reduction means during entry of the vehicle into a turn and an energy regeneration mode which recovers energy while the vehicle exits a turn; and wherein the electronic control means controls the electric motor so as to provide same direction rotative application of torque to the input node during the powered assist mode of the system, and permits the electric motor to generate energy recovered through the speed reduction means from a counter direction rotative application of torque during the energy regeneration mode of the system; wherein the vehicle presents a steering load stiffness to the power steering system; and wherein the electronic control means employs the assist torque signal VTT2 generated by the primary asset torque sensor to counteract an electro-mechanical resonance occurring predominantly between the moment of inertia of the electric motor and a system spring rate presented by the steering load stiffness; wherein the electronic control means treats the electro-mechanical resonance as a resonance between a mass element predominated by the moment of inertia of the rotor of the electric motor and a theoretical spring element dominated by tire loading characteristics associated with the tires; and wherein the electronic control means employs the assist torque signal VTT2 generated by the primary assist torque sensor to control the electric motor so that the speed reduction means is not driven in harmony with the electro-mechanical resonance; wherein the electronic control means provides compensation of at least one of the magnitude of the applied steering torque Ts, and the operation of the electric motor or the assist torque signal VTT2 generated by the primary assist torque sensor, and wherein the compensation of the magnitude of the applied steering torque Ts predominantly includes at least one pole and the compensation of the operation of the electric motor or the assist torque signal VTT2 is predominated by a low-frequency zero; wherein the vehicle includes a steering wheel, such that the application of an applied steering torque Ts to the steering wheel results in the application by the system of a steering force assist to the dirigible wheels; wherein the electronic control means employs the assist torque signal VTT2 generated by the primary assist torque sensor to establish an internal servo control loop between the input signal Vi derived from the application of an applied steering torque Ts to the steering wheel and the steering force assist to the dirigible wheels, and wherein the speed reduction means and electric motor are disconnected from the remaining portions of the power steering system should the powered assist to steering provided by the system fail.
In a seventh aspect, a second alternative preferred embodiment of the present invention is directed to another particular combination of the elements identified above. More particularly, in this seventh aspect, the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a torque summing means having an applied torque input node, an electric motor input node and an output node and adapted to supply a powered assist to steering the dirigible wheels of the vehicle upon the application of torque to the applied torque input node; a primary applied steering torque sensor which generates a signal VTT1 in response to at least an applied steering torque Ts; a primary assisted output torque sensor which generates an assisted output torque signal VTT6 in response to assisted output torque Tao delivered from the output node; an electric motor operatively connected to the electric motor input node and capable of reversibly driving the torque summing means; an electronic control means to which the primary assisted torque sensor and the primary applied steering torque sensor are operatively connected, and which controls actuation of the electric motor; a redundant applied steering torque sensor which generates a redundant applied torque signal VTT3 in response to the applied steering torque Ts and which is connected to the electronic control means; an actuation speed calculation means operatively comprised within the electronic control means for providing a tachometer signal Vt representative of steering movement of the dirigible wheels; a calculated expected by fictitious assist torque signal VTT7 indicative of the expected value for the assisted output torque, and a redundant assisted output torque sensor for providing a redundant assisted output torque signal VTT8 indicative of the actual assisted output torque Tao; wherein the electronic control means establishes servo control over the powered assist to steering supplied by the torque summing means in dependence upon the assisted output torque signal VTT6 generated by the primary assisted output torque sensor; wherein the electronic control means establishes closed loop servo control over the electric motor in dependence upon the assisted output torque signal VTT6 generated by the primary assisted output torque sensor and achieves a substantially linear relationship between the applied steering torque Ts and the powered assist to steering supplied by the torque summing means; wherein the electronic control means generates an error signal Ve in response to at least the generation of the applied torque signal VTT1 by the primary applied steering torque sensor, the error signal Ve being dependent upon the assisted output torque signal VTT6 generated by the primary assisted output torque sensor; wherein the electronic control means determines an internal control loop gain value ICG as a function of at least vehicle speed, the internal control loop gain value ICG being further related to the signal VTT1 generated by the primary applied steering torque sensor; and wherein the electronic control means multiplies the error signal Ve by the internal control loop gain value ICG and causes the electric motor to drive the speed reduction means in a manner which generates an assisted output torque at the primary assisted output torque sensor in accordance with the product of the error signal Ve and the internal control loop gain value ICG; wherein the electronic control means generates the error signal Ve by compensating the signal VTT1 generated by the primary applied steering torque sensor to yield a compensated steering torque signal Vc; subtracting from the compensated steering torque signal Vc a tachometer signal Vt, representative of steering movement of the dirigible wheels, to yield an input signal Vi; obtaining an assisted output torque-dependent internal loop feedback signal Vf and subtracting the feedback signal Vf from the input signal Vi to yield the error signal Ve; wherein the electronic control means terminates the powered assist provided by the system when the redundant applied torque signal VTT3 from the redundant applied steering torque sensor fails to correlate with the applied torque signal VTT1 from the primary applied steering torque sensor; wherein the vehicle includes a steering wheel to which the primary applied steering torque sensor is operatively connected, and wherein a tachometer redundantly measures the rotational speed of at least one of the steering wheel and the electric motor; wherein the electronic control means terminates the powered assist provided by the system when either the calculated expected but fictitious signal VTT7 indicative of the expected assisted output torque or the redundant assisted output torque signal VTT8 indicative of the actual assisted output torque Tao provided by the redundant assisted output torque sensor fails to correlate with the assisted output torque signal VTT6 generated by the primary assisted output torque sensor also indicative of the actual assisted output torque Tao; wherein the system is regenerative and alternatively operable in an unassisted straight steering mode, a powered assist steering mode in which the assisted output torque is provided by the torque summing means during entry of the vehicle into a turn and an energy regeneration mode which recovers energy while the vehicle exits a turn; and wherein the electronic control means controls the electric motor so as to provide same direction rotative application of torque to the electric motor input node during the powered assist mode of the system, and permits the electric motor to generate energy recovered through the torque summing means from a counter direction rotative application of torque during the energy regeneration mode of the system; wherein the vehicle presents a steering load stiffness to the power steering system; and wherein the electronic control means employs the assisted output torque signal VTT6 generated by the primary assisted output torque sensor to counteract an electromechanical resonance occurring predominantly between the moment of inertia of the rotor of the electric motor and a system spring rate presented by the steering load stiffness; wherein the electronic control means treats the electromechanical resonance as a resonance between a mass element predominated by the moment of inertia of the rotor of the electric motor and a theoretical spring element dominated by tire loading characteristics associated with the tires; and wherein the electronic control means employs the assisted output torque signal VTT6 generated by the primary assisted output torque sensor to control the electric motor so that the torque summing means is not driven in harmony with the electromechanical resonance; wherein the electronic control means provides compensation of at least one of the magnitude of the applied steering torque Ts, and the operation of the electric motor or the assisted output torque signal VTT6 generated by the primary assisted output torque sensor, and wherein the compensation of the magnitude of the applied steering torque Ts predominantly includes at least one pole and the compensation of the operation of the electric motor or the assisted output torque signal VTT6 is predominated by a low-frequency zero; wherein the vehicle includes a steering wheel, such that the application of an applied steering torque Ts to the steering wheel results in the application by the system of an assisted steering force to the dirigible wheels; and wherein the electronic control means employs the assisted output torque signal VTT6 generated by the primary assisted output torque sensor to establish an internal servo control loop between the input signal Vi derived from the application of an applied steering torque Ts to the steering wheel and the assisted steering force to the dirigible wheels, and wherein the torque summing means and electric motor are disconnected from the remaining portions of the power steering system should the powered assist to steering provided by the system fail.
In an eighth aspect, a third alternative preferred embodiment of the present invention is directed to yet another particular combination of the elements identified above. More particularly, in this eighth aspect, the present invention is directed to a power steering system for a vehicle having dirigible wheels, comprising: a steering force generating means having an applied torque input node, an electric motor input node and a steering force output node and adapted to supply a powered assist to steering the dirigible wheels of the vehicle upon the application of torque to the applied torque input node; a primary applied steering torque sensor which generates a signal VTT1 in response to at least an applied steering torque Ts; a primary steering force sensor which generates a steering force signal VFT1 in response to steering force Fr delivered from the steering force output node; an electric motor operatively connected to the electric motor input node and capable of reversibly driving the steering force generating means; an electronic control means to which the primary steering force sensor and the primary applied steering torque sensor are operatively connected, and which controls actuation of the electric motor; a redundant applied steering torque sensor which generates a redundant applied torque signal VTT3 in response to the applied steering torque Ts and which is connected to the electronic control means; an actuation speed calculation means operatively comprised within the electronic control means for providing a tachometer signal Vt representative of steering movement of the dirigible wheels; a calculated expected but fictitious signal VFT2 indicative of the expected steering force, and a redundant steering force sensor for providing a redundant steering force signal VFT3 indicative of the actual steering force Fr; wherein the electronic control means establishes servo control over the powered assist to steering supplied by the steering force generating means in dependence upon the steering force signal VFT1 generated by the primary steering force sensor; wherein the electronic control means establishes closed loop servo control over the electric motor in dependence upon the steering force signal VFT1 generated by the primary steering force sensor and achieves a substantially linear relationship between the applied steering torque Ts and the powered assist to steering supplied by the steering force generating means; wherein the electronic control means generates an error signal Ve in response to at least the generation of the applied torque signal VTT1 by the primary applied steering torque sensor, the error signal Ve being dependent upon the steering force signal VFT1 generated by the primary steering force sensor; wherein the electronic control means determines an internal control loop gain value ICG as a function of at least vehicle speed, the internal control loop gain value ICG being further related to the signal VTT1 generated by the primary applied steering torque sensor; and wherein the electronic control means multiplies the error signal Ve by the internal control loop gain value ICG and causes the electric motor to drive the steering force generation means in a manner which generates a steering force Fr at the primary steering force sensor in accordance with the product of the error signal Ve and the internal control loop gain value ICG; wherein the electronic control means generates the error signal Ve by compensating the signal VTT1 generated by the primary applied steering torque sensor to yield a compensated steering torque signal Vc; subtracting from the compensated steering torque signal Vc a tachometer signal Vt, representative of steering movement of the dirigible wheels, to yield an input signal Vi; obtaining an steering force-dependent internal loop feedback signal Vf and subtracting the feedback signal Vf from the input signal Vi to yield the error signal Ve; wherein the electronic control means terminates the powered assist provided by the system when the redundant applied torque signal VTT3 from the redundant applied steering torque sensor fails to correlate with the applied torque signal VTT1 from the primary applied steering torque sensor; wherein the vehicle includes a steering wheel to which the primary applied steering torque sensor is operatively connected, and wherein a tachometer redundantly measures the rotational speed of at least one of the steering wheel and the electric motor; wherein the electronic control means terminates the powered assist provided by the system when either the calculated expected but redundant signal VFT2 indicative of the expected steering force or the redundant steering force signal VFT3 indicative of the actual steering force Fr provided by the redundant steering force sensor fails to correlate with the steering force signal VFT1 generated by the primary steering force sensor also indicative of the actual steering force Fr; wherein the system is regenerative and alternatively operable in an unassisted straight steering mode, a power assisted steering mode in which the steering force is provided by the steering force generating means during entry of the vehicle into a turn and an energy regeneration mode which recovers energy while the vehicle exits a turn; and wherein the electronic control means controls the electric motor so as to provide same direction rotative application of torque to the electric motor input node during the powered assist mode of the system, and permits the electric motor to generate energy recovered through the steering force generating means from a counter direction rotative application of torque during the energy regeneration mode of the system; wherein the vehicle presents a steering load stiffness to the power steering system; and wherein the electronic control means employs the steering force signal VFT1 generated by the primary steering force sensor to counteract an electromechanical resonance occurring predominantly between the moment of inertia of the rotor of the electric motor and a system spring rate presented by the steering load stiffness; wherein the electronic control means treats the electromechanical resonance as a resonance between a mass element predominated by the moment of inertia of the rotor of the electric motor and a theoretical spring element dominated by tire loading characteristics associated with the tires; and wherein the electronic control means employs the steering force signal VFT1 generated by the primary steering force sensor to control the electric motor so that the steering force generating means is not driven in harmony with the electromechanical resonance; wherein the electronic control means provides compensation of at least one of the magnitude of the applied steering torque Ts, and the operation of the electric motor or the assisted output torque signal VFT1 generated by the primary steering force sensor, and wherein the compensation of the magnitude of the applied steering torque Ts predominantly includes at least one pole and the compensation of the operation of the electric motor or the steering force signal VTT6 is predominated by a low-frequency zero; wherein the vehicle includes a steering wheel, such that the application of an applied steering torque Ts to the steering wheel results in the application by the system of an assisted steering force Fr to the dirigible wheels; and wherein the electronic control means employs the steering force signal VFT1 generated by the primary steering force sensor to establish an internal servo control loop between the input signal Vi derived from the application of an applied steering torque Ts to the steering wheel and the assisted steering force Fr to the dirigible wheels, and wherein the electric motor is disconnected from the steering force generating means should the powered assist to steering provided by the system fail.
The power steering systems of the present invention possess several distinct advantages over prior EPS systems. For example, the systems of the present invention provide fall time independent verification of instant values of steering boost, substantially eliminating concerns relating to auto-steer. The systems of the present invention also substantially eliminate low frequency stability control problems previously encountered in EPS systems. Advantageously, the steering force outputs from the systems of the present invention are linearly related to the applied steering torque.
Of course, the systems of the present invention enjoy all of the advantages of a regenerative system. The power steering systems of the present invention possess high efficiency and recover a significant portion of the energy returned from the dirigible wheels when the host vehicle exits a turn. More particularly, the ability to handle regenerative power allows the systems of the present invention to operate in all four quadrants which permits smooth transition between powered assist and energy regeneration modes, and to permit good steering control both during operation in each of the modes and during transition between modes. Finally, the systems of the present invention allow safe operation of the vehicle during failure of the powered assist provided by the systems.